(a) Field of the Invention
The present invention relates to thin film transistor (TFT) panel for a liquid crystal display (LCD) and a method for manufacturing the same.
(b) Description of the Related Art
An LCD is one of the most popular flat panel displays (FPD). The LCD has two panels having two kinds of electrodes for generating electric fields and a liquid crystal layer interposed there between. The transmittance of incident light is controlled by the intensity of the electric field applied to the liquid crystal layer.
The field-generating electrodes may be formed at each of the panels, or at only one of the panels. A panel with at least one kind of electrode has switching elements, such as thin film transistors.
In general, a TFT array panel of an LCD includes a plurality of pixel electrodes and TFTs controlling the signals supplied to the pixel electrodes. The TFT array panel is manufactured by photolithography using a plurality of photomasks, and it undergoes five or six photolithography steps before it is completed. The high costs and lengthy time required for the photolithography process makes it desirable to reduce the number of the photolithography steps.
Several manufacturing methods of LCDs using only four photolithography steps have been suggested, such as that in Korean Patent Application No. 1995-189 ('189). However, as an LCD actually requires wires for transmitting electric signals to the TFTs and wire pads for receiving external signals, the full process to complete a TFT array panel requires the step of forming such pads. Unfortunately, '189 does not disclose how to form such pads.
Another conventional method of manufacturing a TFT array panel using only four photolithography steps is disclosed in "A TFT Manufactured by 4 Masks Process with New Photolithography (Chang-wook Han et al., Proceedings of The 18th International Display Research Conference Asia Display 98, pp. 1109-1112, 1998. 9.28-10.1).
Furthermore, a storage capacitor for sustaining the voltage applied to a pixel is generally provided in the TFT array panel, and the storage capacitor includes a storage electrode and a portion of a pixel electrode as well as a passivation layer interposed there between. The storage electrode is made of the same layer as a gate wire, and a portion of the pixel electrode is formed on the passivation layer. The storage electrode is covered with a gate insulating layer, a semiconductor layer, and a passivation layer, with most of the pixel electrode being formed directly on the substrate in Han et al. Therefore, the pixel electrode should be stepped up over the triple layers of the gate insulating layer, the semiconductor layer, and the passivation layer in order to overlap the storage electrode. This may result in a disconnection of the pixel electrode in the vicinity of a high step-up area.
As shown in '189, conventional photolithography processes uses a photoresist (PR) layer. The conventional photoresist layer is exposed to light through a photomask and thereby divided into two sections, that is, the part exposed to the light and the other part that is not so exposed. The development of the photoresist layer forms the PR pattern having a uniform thickness once the PR layer exposed to the light has been completely removed. Accordingly, the etched thickness of the layers under the PR pattern is also uniform. However, Han et al. uses a photomask having a grid, which lowers the amount of light reaching the portion of a positive PR layer thereunder in order to form a PR pattern having some portions thinner than other portions. The different thicknesses of the PR pattern produces the different etching depths of the underlying layers.
However, the method of Han et al. has a problem in forming the grid throughout a wide region. Furthermore, it is hard to make the etching depth uniform under the grid region, even when the grid is formed throughout a wide region.
U.S. Pat. Nos. 4,231,811, 5,618,643, and 4,415,262 and Japanese patent publication No. 61-181130, etc., which disclose similar methods as do Han et al. also have the same problem.